I've never seen a definition, but I've always thought of a Zwischenzug as being a move in the middle of a sequence of exchanges. Maybe it's just me. Whatever you call it, it's a very nice finish. Not hard to see if you know something is there, but very easy to miss otherwise!

That's why I see it as an inbetween move. The key idea is Be8 interfering with the Rook's defence of f8 - but it doesn't work straight away because the King is too close. So Black has to play Bf7 first, chasing the King away and only then continue on it's journey to e8.

So the Bishop stops off on f7 inbetween moving from h5 and e8. Hence, to me, zwischenzug

Put it this way: Would you see it as a zwischenzug if the solution was, say, Bf7 and Bb3? Or Bf7 and Bg8?

Alternatively remove the black queen and white bishop from the board. 1. Qf7+ and 2.Qxf8 Zwischenzug? Not really. A zwischenzug is more a move inserted into the middle of a forced sequence (not necessarily with exchanges) but independent of that sequence (other than that it changes the assessment of the position at the end of it). It's certainly not a sequence (Bf7, Kh8) inserted within a move!

For me the two themes here are "deflection" and "interference". Bf7 is deflection and Be8 is interference.

"The zwischenzug is a common chess tactic in which a player, instead of playing the expected move (commonly a recapture of a piece that the opponent has just captured) first interpolates another move, posing an immediate threat that the opponent must answer, then plays the expected move. Ideally, the zwischenzug changes the situation to the player's advantage. Such a move is also called an intermezzo, intermediate move, or in-between move."

It also says this game was described by Chernev no less as "The Immortal Zwischenzug" game. Since the game is far less interesting from the one from the same tournament on the blog post, I think we can now be pretty sure that the above definitely is not a Zwischenzug...

The chessworld.net definition is a bit clumsy, but to me also communicates the spirit of Z (even if the word 'capture' should be replaced by 'threat', and 'recapture' by something about not answering the threat directly.)

"A move played in response to a capture which is not a recapture, but which forces the opponent to make a reply which cannot avoid eventual capture."

Also I just went through Paul Littlewood's chapter on Z in his (excellent) book "Chess Tactics". The examples all seem to involve doing something in the middle of a sequence, either unexpectedly or in not answering the opponent's threat.

Btw, MegaBase 2007 has many examples of Z classified, if you look the term up in the database's 'themes'.